I would like to know how bluetooth works with Android : when you turn bluetooth on, is it broadcasting data that other devices can detect ? If not, is it possible to detect from a BLE device if a specific Android with the bluetooth turned on is nearby ? And how ? Thanks.
It would be possible with "classic" Bluetooth, but not with Bluetooth Low Energy (BLE). In BLE, the peripheral device (the beacon, keyboard, or whatever) broadcasts adverts. The central device (the phone) just listens for those adverts.
The central device only sends anything when it wants to connect to a particular BLE peripheral. You can detect that using a BLE sniffer device, but not with an ordinary BLE peripheral (other than the one the phone is connecting to), because the BLE hardware itself filters out packets that aren't intended for that peripheral.